Testing feedback amplifiers



April 1940- l. G. WILSON 2,195,439

TESTING FEEDBACK AMPLIFIER Filed Dec. 9, 1937 PHASE AND GAIN FREQ. KC

INVENTOR 6. WILSON UM y ATTOPA e7 Patented Apr. 2, 1940 v 1 UNITEDSTATES. PATENT. OFFICE i s i 2.19'5,4 s9 r TESTING FEEDBACK AlVHLlIFIEBS,Ira'G. Wilson, New York, N.[Y., assignor to Bell TelephoneLaboratories, Incorporated, New York, Y., a: corporation of lslew York 1Application December 9.1937, Serial No; 178,899

2 Claims.- (Ql.[179-171). 1'

1 The present invention relates to the testing of amplifiers todetermine their singing margin.

In the design of amplifiers provided withfeedback it is often adifficult matter to design the circuit to have high gain throughoutadesired frequency band and at the same time a suflicient margin againstself-oscillation or sing'ing. In the manufacture of such amplifiers itis desirable to test' them to determine how great a singing 7 marginthey possess before putting them out for use. v

A criterion for determining whether ornot a "given amplifier will bestable or. unstable as regards singing tendency was devised by H.Nyquist,

and is disclosed in his Patent No. 1,915,449, granted June 2'7, 1933.After'that criterion was made available, it became customary in thelaboratory to make a series of measurements onan amplifier throughoutthe frequency range'of in terest in order to determine thecharacteristic curve proposed by Nyquist and in that way determinewhether or not a given amplifier was stable or unstable as regardssinging-tendency.

While this method is suitable for laboratory studies, it would be costlyand time-consuming as 'a manufacturing test.

The present invention provides a simple test which can be made quicklyand whichgives .a quantitative measure of the minimum singing margin ofan amplifier. The method and means provided by the invention aresuitable for making routine tests of amplifiers in manufacture or forother purposes.

The invention will now be described in greater detail with the aid ofthe accompanying draw-' ing.

In the drawing, Fig. l is a simplified schematic diagram of a typicalstabilized'feedback amplifier with testing networks in accordance withthe invention adapted to be connected to it; and

Fig. 2 shows gain and phase curves for the amy plifier, to be referredto in the description.

In Fig. 1, there is shown by way of illustration, a three-stageamplifier comprising tubes l, 2, and 3 connected in tandem with an inputcircuit at 10 and an output circuit at H. Interstage networks suitablydesigned with respect to the frequency range to be transmitted areindicated at 4 and 5. The output of stage 3 is illustrated as containinga bridge M of which the feedback diagonal is shown at l5. The feedbackconnection extends from slider "5 on resistance l5 through gain controlcondenser l1 and circuit [8 to a terminal of resistance 2| in the gridcircuit of stage I. Resistance 2| should be ing the properphasecharacteristic.

will be understood as the description proceeds amplifiers used forbroadband amplification in multiplex carrier telephone systems wherehigh gain characteristics of predetermined type are required. Theamplifier may contain suitable equalizers, for example, in the feedback(not shown) or other means for suitably shaping'the waves. Cathodenetworks I2 and I3 are shown in connection with tubes I and 2 forfurnishing. grid bias and, if desired, for assisting in obtain- Thevolt: age sources for energizing the amplifier have, for simplicity,been "omitted from the drawing except that the space current source 2'7is shown for supplying the" space current to the tube 3 in series withspace current meter 28. A more complete disclosure of this type; ofamplifier may be found in the copending application of H. S.

' Black Serial No. 606,871, filed April 22, 1932, 'Patent No. 2,102,671,granted December 21, 1937.. It

25 thatthe invention is not dependent upon the detailed design of thefeedback amplifier with which the invention is to be used so that theamplifier shown in Fig. 1 may be considered as merely. typical. v 1 i Ifthe gain and phase around the amplifier and. feedback loop in anamplifier circuit 'such asthat illustrated in Fig. 1 fall within certainvalues, as more fully disclosed in the Nyquist patent supra, theamplifier will develop self-oscillations or singing. There are certainportions of the frequency range of a broad band amplifier in which thegain and phase around the amplifier feedback loop approach most closelythe values correspending to the singing condition. Consequent- 4o ly,the singing margin of the amplifier can be tested by determining how farthe gain and phase at these frequencies are from the values of the gainand phase corresponding to the singing condition. q In accordance withthe present invention, the singing margin may be determined by use ,ofeither test net A or test net B shown adjacent the amplifier of Fig. 1.When test net A is used,

along the feedback circuit. When the test net 55 ticularly in makingroutine or manufacturing.

denser 25, it is connected. in shunt with the condenser II, as forexample by clipping its two terminals to the leads I6 and i8 on oppositeI sides of condenser 11. The resistances and condensers in the test netsare shown variable. In general, the efiect of varying the capacity is tovary the amount of the insertion phase shift and the efiect ofvarying'the resistance" isto vary the frequency at which the phase shifttakes place. It may be more convenient, however, parof use of the testnets may now be described in connection with the curves ofFig. 2.

In Fig. 2 the gain and phase characteristics C and D, respectively, forthe amplifier of Fig. 1

are plotted over the portion of the frequency range within which theamplifier has the greatest singing tendency. This is the region in whichthe gain and phase characteristics approach the asymptotic values and isgenerally at some frequency high in comparison with the utilized range.the neighborhood of 10 to 100 kilocycles, the frequency of greatestsinging tendency may be in the neighborhood of 600 to 1290 kilocycles.(This problem is more fully discussed in Robertson Patent 1,994,486,March19, 1935 to whichreference may be made.)

Referring to the curves of Fig. 2, if the phase characteristic 1) of theamplifier should dip below the zero line at any frequency for whichthegain 0 is positive, singing would result. If the phase curve D ispositive at all frequencies for which the gain curve'C is positive, thecircuit is not in singing condition. However, the phase curve D shouldnot become negative until frequencies are reached considerably higherthan the frequency corresponding to the cross-over point of the gaincurve. The singing margin may be tested by deliberately degrading thephase by predetermined amounts and noting whether the amplifier singsunder these conditions. In practice this maydone by providing two testnets similar, for example, to test net B, for degrading the phase by adefinite amount at each of two frequencies, such as 1200 kilocycles and600 kilocycles, at which singing is most likely to occur. Typical curvesshowing the results of such tests are given at E and F. Curve E showsthe position develops. For example, if the utilized band is in to whichthe phase curve D was shifted by connecting test net B to the positionshown in Fig. 1. Curve F shows the position to'which the phase curve Dwas shifted when test net B was used at the position shown with othervalues of resistance and capacity. Since the space current meter 28showed no deflection when either test was applied, it was determined bythese tests that the amplifier possessed a singing margin at least asgreat as that amounting to the phase shift introduced by the testingnet. Similar results could have been obtained by use of test nets atposition A.

Variations of the test method of the invention will suggest themselvesto those skilled in the art. The test networks can be connected to otherpoints of the circuit than those shown, for

example, across the cathode network. A test net may be connected to thecircuit and its resistance or'capacity, or both may be varied to sweepover a range of frequency and phase while noting space current meter 28or any other suitable indicator to determine whether or not a singingcondition The method, however, lends itself readily to a routine test inwhich one or more networks of predetermined characteristic are clippedon to the circuit. and the singing or non-singing condition noted bywatching the space current.

meter. This affords a simple acceptance-rejection test for amplifiershaving sufficient or insufficient singing margin.

What is claimed is:

1. The method of testing a feedback amplifier for'singing margincomprising arbitrarily varying the phase shift around the feedback loopby a given amount in the direction toward zero phase shift at differentportions of the total frequency range which the amplifier is capable oftransmitting and determining whether or not the amplifier will sing as aresult of such phase variations.

2. The methodof quantitatively testing a negative feedback amplier forsinging margin, which method consists in temporarily degrading the phasemargin by a predetermined amount at different portions of the totalfrequency range which the amplifier is capable of transmitting, therebyincreasing by a definite amount its singing tendency and determiningwhether such degradation in phase margin causes the amplifier to sing.

IRA G. wmsoN.

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